Work and EnergyWork and Energy

VocabularyVocabulary Work: Work: The use of force to move an object over a The use of force to move an object over a

distance (Joule or J)distance (Joule or J)

Joule: Joule: The unit used to measure work; one joule of The unit used to measure work; one joule of work is done when a force of one newton moves an work is done when a force of one newton moves an object one meter (Joule = Newton object one meter (Joule = Newton ● meter)● meter)

Power:Power: the rate at which work is done (Watts) the rate at which work is done (Watts)

Watt:Watt: the unit used to measure power; one watt is equal the unit used to measure power; one watt is equal to one joule of work done in one secondto one joule of work done in one second

Horsepower: Horsepower: the unit of measurement for engines and the unit of measurement for engines and motors; one horsepower equals 745 watts.motors; one horsepower equals 745 watts.

WORKWORK Force is necessary to do work!Force is necessary to do work!

Work is done only when an object that is being Work is done only when an object that is being pushed or pulled actually moves.pushed or pulled actually moves.

The work done by a force is related to the size of The work done by a force is related to the size of the force and the distance over which the force is the force and the distance over which the force is applied.applied.

Work= (Force) Work= (Force) ● (distance) W= Fd● (distance) W= Fd

Units: Work (Joules), Force (Newtons), Units: Work (Joules), Force (Newtons), Distance (Meters) Distance (Meters)

Calculating Work (W=Fd)Calculating Work (W=Fd)

How much work is done if a person lifts a How much work is done if a person lifts a barbell weighing 500 N to a height of 2 barbell weighing 500 N to a height of 2 meters?meters?

IIFFSSMMUU

Calculating WorkCalculating Work I: Force= 500 N, distance= 2 m, Work=?I: Force= 500 N, distance= 2 m, Work=?

F: Work= ForceF: Work= Force●distance (W=Fd)●distance (W=Fd)

S: W= 500 N ● 2 mS: W= 500 N ● 2 m

M: 1,000M: 1,000

U: 1,000 J U: 1,000 J

(remember the units for work are joules, units for (remember the units for work are joules, units for force are Newtons, and units for distance are force are Newtons, and units for distance are meters)meters)

WorkWork

Objects that are moving can Objects that are moving can do workdo work

Examples:Examples:

Bowling balls do work on the Bowling balls do work on the pins they hit in a bowling alleypins they hit in a bowling alley

Baseball bat does work on the Baseball bat does work on the ballball

Gravity does work on falling Gravity does work on falling waterwater

Energy is transferred when Energy is transferred when work is donework is done

Energy is the ability to do work or cause Energy is the ability to do work or cause changechange

When you do work on an object, some of your When you do work on an object, some of your energy is transferred to the object.energy is transferred to the object.

Think of work as a transfer of energyThink of work as a transfer of energy

Work and energy are both measured in the Work and energy are both measured in the same unit, the joulesame unit, the joule

Calculating Gravitational Calculating Gravitational Potential EnergyPotential Energy

Potential energy (stored energy) that is Potential energy (stored energy) that is caused by gravity (downward force) is caused by gravity (downward force) is gravitational potential energy. (GPE)gravitational potential energy. (GPE)

GGravitational ravitational ppotential otential eenergy= nergy= mmass ass ● ● ggravitational acceleration ● ravitational acceleration ● hheighteight

(GPE = mgh)(GPE = mgh)

Calculating Gravitational Calculating Gravitational Potential Energy Potential Energy (GPE=mgh)(GPE=mgh) What is the gravitational potential energy of a girl who has a What is the gravitational potential energy of a girl who has a mass of 40 kg and is standing on the edge of a diving board mass of 40 kg and is standing on the edge of a diving board that is 5 meters above the water? that is 5 meters above the water?

* remember gravitational acceleration is a constant (does * remember gravitational acceleration is a constant (does not change) so…not change) so…

g= 9.8 m/sg= 9.8 m/s22

II FF SS MM UU

Calculating Gravitational Calculating Gravitational Potential EnergyPotential Energy

I: mass= 40 kg, g= 9.8 m/sI: mass= 40 kg, g= 9.8 m/s2 2 , height =5 m, GPE=?, height =5 m, GPE=?

F: GPE = mghF: GPE = mgh

S: GPE= 40 kg S: GPE= 40 kg ● ● 9.8 m/s9.8 m/s2 2 ●● 5 m 5 m

M: 1960 (kg ● M: 1960 (kg ● m/sm/s2 2 ) ) ● m● m

U: 1960 JoulesU: 1960 Joules

Remember the unit (Joules = N ● m)Remember the unit (Joules = N ● m)

Calculating Kinetic Energy Calculating Kinetic Energy KE= KE= mvmv22

2 2 Kinetic energy is the energy of motionKinetic energy is the energy of motion

Kinetic energy= Kinetic energy= mass mass ● velocity● velocity22

22

Calculating Kinetic Energy Calculating Kinetic Energy KE= KE= mvmv22

2 2 What is the kinetic energy of a girl who has a What is the kinetic energy of a girl who has a

mass of 40 kg and a velocity of mass of 40 kg and a velocity of

3 m/s3 m/s

II

FF

SS

MM

UU

Calculating Kinetic Energy Calculating Kinetic Energy KE= KE= mvmv22

2 2 I: mass= 40 kg, velocity = 3 m/sI: mass= 40 kg, velocity = 3 m/s , KE=? , KE=?

F: KE= F: KE= mvmv22

2 2 S: KE= S: KE= (40kg (40kg ● (● (3m/s )3m/s )22

22

■ ■ M: M: (40kg (40kg ● 9 ● 9 mm22/s/s22 )) 22

360 360 22

180 ( kg ● m180 ( kg ● m22/s/s2 2 ) ) (kg ● m/s(kg ● m/s22 ) ● m ) ● m N ● mN ● m

180 N ● m180 N ● m

■■U: 180 J (Remember N ● m = J)U: 180 J (Remember N ● m = J)

Calculating Mechanical Calculating Mechanical Energy ( ME= PE + KE)Energy ( ME= PE + KE)

Mechanical energy is the object’s Mechanical energy is the object’s combined potential and kinetic energycombined potential and kinetic energy

Mechanical energy= potential energy + Mechanical energy= potential energy + kinetic energykinetic energy

ME = PE + KEME = PE + KE

Calculating Mechanical Calculating Mechanical Energy ( ME= PE + KE)Energy ( ME= PE + KE)

The potential energy of a girl on a swing just The potential energy of a girl on a swing just before she is released is 180 J and her kinetic before she is released is 180 J and her kinetic energy after she has been released is 300 J. energy after she has been released is 300 J. What is her mechanical energy (total energy) ?What is her mechanical energy (total energy) ?

II FF SS MM UU

Calculating Mechanical Calculating Mechanical Energy ( ME= PE + KE)Energy ( ME= PE + KE)

I: PE= 180 J, KE= 300 J, ME=?I: PE= 180 J, KE= 300 J, ME=?

F: ME= PE + KEF: ME= PE + KE

S: ME= 180 J + 300 JS: ME= 180 J + 300 J

M: 480M: 480

U: 480 JU: 480 J

Law of conservation of Law of conservation of energyenergy Energy can not be created or destroyed it Energy can not be created or destroyed it

is just transferred or transformedis just transferred or transformed

No matter how much energy is transferred No matter how much energy is transferred all of the energy is still present somewhere all of the energy is still present somewhere in one form or anotherin one form or another

Mechanical energy is the combination of Mechanical energy is the combination of both kinetic and potential energy.both kinetic and potential energy.

Power Power

Power is the rate at Power is the rate at which work is donewhich work is done

The unit of The unit of measurement for measurement for power is power is watt (W)watt (W)

Power = Power = WorkWork

TimeTime

P= W/tP= W/t

Calculating Power from Calculating Power from work ( P= W/t)work ( P= W/t)

An Antarctic explorer uses 6000 J of An Antarctic explorer uses 6000 J of work to pull his sled for 60 Seconds. work to pull his sled for 60 Seconds. What power does he need?What power does he need?

II FF SS MM UU

Calculating Power from Calculating Power from work ( P= W/t)work ( P= W/t)

I: Work= 6000 J, time= 60 s , P=?I: Work= 6000 J, time= 60 s , P=? F: P= W/tF: P= W/t S: P= 6000J/ 60 sS: P= 6000J/ 60 s M: P=100 J/s M: P=100 J/s ( J/s =Watts)( J/s =Watts) U: P=100 WU: P=100 W

Power can be calculated Power can be calculated from energy (P=E/t)from energy (P=E/t)

Power = Energy/timePower = Energy/time A light bulb used 600 J of energy in 6 s. A light bulb used 600 J of energy in 6 s.

What is the power of the light bulb?What is the power of the light bulb? II FF SS MM UU

Power can be calculated Power can be calculated from energy (P=E/t)from energy (P=E/t)

I: Energy= 600 J, Time= 6 s, P=?I: Energy= 600 J, Time= 6 s, P=? F: P=E/tF: P=E/t S: P= 600 J/6 sS: P= 600 J/6 s M: P= 100 J/sM: P= 100 J/s U: P= 100 WattsU: P= 100 Watts